Displacement ventilation systems are in wide spread use in the HVAC industry. One particular application for such systems is in school buildings. Typically, a number of displacement air terminal devices will be located along a perimeter wall of a classroom space and are configured to deliver ventilation air to the space. The ventilation air is generally several degrees below the space set point temperature to ensure that the ventilation air effectively pools across the floor of the space. Due to the heat generated by the occupants and equipment in the space, the ventilation air is naturally drawn up by convection which ensures fresh air is continually delivered to the breathing zone of the occupants. The heated air continues to rise to the ceiling level where it can be removed from the space. Thus, those skilled in the art readily recognize the advantages of displacement ventilation air systems to both cool and ventilate a space in a highly efficient manner.
It is also known to configure displacement air terminal devices to have induction nozzles such that room air from the space can be conditioned, filtered, and returned to the space in a mixture with the ventilation air. This type of unit is sometimes referred to as an induction-displacement unit. One such unit is sold as the “Trox QLCI Displacement Terminal” and is available from Trox USA, Inc. of Alpharetta, Ga. A similar unit and system are described in U.S. Pat. No. 7,013,969 to Loudermilk, the entirety of which is hereby incorporated by reference herein. In this type of system, a coil is provided in the return air path of the induction-displacement unit to condition the return air, as necessary. In the cooling mode, the coil will reduce the return air temperature to a few degrees below the space temperature to match the temperature of the ventilation air. In the heating mode, the coil may be set to raise the temperature of the return air such that the delivered air has a higher temperature than that of the air in the room. In this mode a minimum space temperature set point can be maintained. However, in doing so, the induction-displacement unit can no longer function as a displacement terminal unit due to the fact that the delivered air starts rising as soon as it leaves the unit instead of pooling across the floor. To prevent the necessity of having to place an induction-displacement unit in the heating mode, a separate heating system can be provided in the space, such as fin-tube radiation or radiant panels. However, as the induction-displacement units are placed along at least one wall of the space, such heating systems cannot be placed in the same location. This is especially troublesome where the induction-displacement units are located along an exterior wall of the space because this is the location where heating is generally most needed. As such, improvements are desired for induction-displacement units that can simultaneously provide heating to an exterior wall of a space and provide displacement ventilation air to the space.